There is at the present time a tendency, in the design of motor vehicle headlights, to design the reflector of the headlight in such a way that it will produce by itself a beam having the desired photometric distribution. This photometric distribution consists in particular of giving the beam the required width with satisfactory homogeneity, that is to say, as far as possible, a beam without any undesirable bright spots or dim spots, and also to give the beam the required vertical distribution by delimiting the beam as necessary with a cut-off of a type which can be varied. Such cut-offs exist in particular for European dipped beams, American dipped beams, foglight beams, and so on.
This particular design of the reflector, which has begun to appear on motor cars and other touring vehicles, enables a cover lens to be used which may be entirely smooth or nearly smooth, that is to say it only has decorative elements such as fillets and the like, which play no significant part in the structure of the beam produced by the headlight.
In combination, these characteristics have two main advantageous effects. The first of these is that downward deflection of the light is avoided, the light being widely spread by conventional optical striations where they are formed on a cover lens which is sharply inclined in its vertical axial sections. The second of these advantages is that an aesthetically pleasing headlight is obtained.
However, most recently there has been a tendency to make use of striated cover lenses, particularly with a view to masking as far as possible any defects existing in the material of the lens itself (which may be of glass or of a plastics material), or in the surface of the reflector. Such defects may consist of varnishing faults, the presence of dust, slight scratches, and so on. At the same time, it remains desirable to retain the same reflector design, because this enables beams to be produced which give extreme satisfaction in terms of the photometric distribution obtained.
A difficulty does however arise from the fact that if, in a headlight of the prior art, striations are applied on a cover lens which is placed in front of such a reflector, the photometric qualities of the beam will be unfailingly affected detrimentally.